LUFKIN, Texas — A 39-year-old East Texas kindergarten teacher has been sentenced to 10 years in prison after pleading guilty to seven counts of sexual assault with a child and improper relationship with a student for having sex with high school students.

A judge in Angelina County also has ordered Heather Robertson, of Lufkin, to surrender her teaching certification and register for life as a sex offender.

Robertson was a teacher in the Hudson school district, just outside Lufkin, for 13 years. Prosecutors said Thursday an investigation showed she had sex at her Lufkin apartment with at least six Hudson students between last November and February this year.

She resigned as a Hudson teacher April 20 and was arrested the next day. She could have received up to 20 years on each count.